A power transmission path of the engine of a wheel loader is provided with a transmission having a forward clutch, a reverse clutch, and respective speed gear clutches.
For example, when a working vehicle is moving backward and a forward/reverse selecting lever which is provided at a driver seat is operated to a forward position, a gear change command is generated, the forward clutch is selected, switching is made from the reverse clutch before the gear change to the selected forward clutch after the gear change, the engine power is transmitted to the selected gear-changed forward clutch of the transmission to drive the drive wheels, and the vehicle is changed to move from reverse to forward.
The wheel loader often performs V-shape loading. The V-shape loading is an operation pattern that the wheel loader travels forward to a natural ground to excavate earth and sand, travels backward after the excavation to reach a turning point, travels forward to load the earth and sand on a hopper or a dump truck and travels the same route repeatedly.
At the time of the V-shape loading, the wheel loader is driven at a low speed and frequently switched to change from reverse running to forward running or from forward running to reverse running.
The troublesome forward/reverse changeover at low-speed running is performed not only at the time of the V-shape loading but also at the time of loading a cargo on a ship by the wheel loader.
When working at a low-speed forward/reverse switching as described above, a low-speed gear range suitable for the work is selected by a shift range lever. For example, the wheel loader adopting a transmission consisting of first to fourth speed gears selects a shift range “2” which is a low-speed gear range to perform low-speed working. When the shift range “2” is selected, automatic gear change is performed with the second speed gear determined as the maximum speed gear in the shift range.
When the shift range “2” is selected and the vehicle speed exceeds the upper limit vehicle speed of the second speed gear which is the maximum speed gear of the shift range “2”, automatic gear change is performed to a speed gear, e.g., a third speed gear, higher than the maximum speed gear (second speed gear) in the shift range in order to protect the clutch of the transmission.
Patent Literature 1 below describes an invention which produces a braking force of the brakes in conjunction with engagement of the clutch at a time of forward/reverse changeover to prevent a gear change shock produced by the clutch.
Patent Literature 1: JP-A 06-92162